RATIONALE: Drugs used in chemotherapy, such as gemcitabine hydrochloride and oxaliplatin, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Observation is watching a patient's condition but not giving treatment until symptoms appear. It is not yet known whether giving gemcitabine hydrochloride together with oxaliplatin is more effective than observation in treating patients with biliary tract cancer that has been removed by surgery.